1. Field of the Invention
The present invention relates to a twister for producing a twisted wire by twisting plural wire materials, and a method for producing a twisted wire, a ply, and a pneumatic tire. More particularly, the present invention relates to a twister that is suitable for producing a tire reinforcing cord for use in a tire, a method for producing a twisted wire, a ply, and a pneumatic tire.
2. Description of the Background Art
In conventional mass production of cord products, such as steel cords, basically, twisters are used to continuously twist wires and the resulting product (cord) is taken up on a reel. When several solid wires are twisted together to produce a cord, it is necessary to make twisting rotation at both sides of a twisting point of the wires along a progressing direction of the wires. Therefore, it is necessary to rotate one of the material (i.e., the solid wires) side and the product (i.e., the cord) side of the wires in the same direction of the twisting rotation, or alternatively, it is necessary to make one of the solid wires and the cord supported within a rotating body and advance along the rotating body rotating around the wires or the cord, so that the one of the solid wires and the cord is twisted around the other of the solid wires and the cord (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 6-200491 and 9-291488).
Further, in tire production, a sheet-like tire intermediate material is made, in which steel cords are embedded in a barred lattice pattern in a rubber sheet. It is thus desired that the steel cord used as a tire reinforcing material is as long and continuous as possible, and is generally produced to have a length of several thousand meters to tens-of-thousands of meters.
A method is known, where a product (twisted wire) is produced by repeatedly inverting a twisting direction at certain intervals with opposite ends being fixed, and therefore unrotated. However, in this method, although each area that is shorter than the interval of the inversion remains twisted, there are inevitably sections where the wires are not twisted between the inversion intervals. In addition, since the twist of the product is cancelled as a whole by the repeated inversion, this method does not produce a good twisted wire.
In order to produce a cord, it is necessary to twist wires as many times as possible within a short time.
However, since only a single or double twisting can be made by a single rotation of a device (a single rotation of a rotating body), there arises a problem that a number of devices (twisters) are necessary. The equipment investment for the devices of this type actually constitutes a large portion of cord production costs.
It is desirable that a twister can rotate at a high-speed. There is a natural limit in the rotation speed of the twister, due to the durable load of a centrifugal force resulting from high-speed rotation and rotational vibration. If a twister is made compact and a rotation frequency thereof is increased to reduce the equipment cost, a winding size of a bobbin is decreased and the bobbin must be replaced more frequently, resulting in lower efficiency. In addition, there is also a limit in size reduction in view of ensuring continuity during the tire production process.
In contrast, if a winding size of a bobbin is increased to reduce required operation steps, a twister becomes huge and a centrifugal force is increased, and therefore a rotation speed cannot be increased, resulting in increase in the equipment cost. Due to this problem, a size and a rotation speed of a twister are set at compromising values in consideration of a balance therebetween, and this makes it difficult to reduce production costs further from the current level.
In addition, since it is normal practice to continuously produce a product having a length of several thousands to several ten-thousands meters, if a problem such as a wire breaking occurs during production, it is necessary to remove a defective product, whose length does not reach a predetermined length, through specific operations such as scrapping or welding. That is, a significant number of extra processes are required to recover the normal state of production.
In tire production, a large quantity of steel cords are processed at the same time with a large calendaring machine or stelastic. Therefore, a process lot is large and this leads to potential problems of increase in stock and increase in a lead time from production to shipment. Further, since it is troublesome to change properties of steel cords for each tire product and/or for each portion of a tire (this would result in low production efficiency), production of diverse products has been inhibited.
In addition, since a continuous steel cord is generally produced and supplied in conventional steel cord production processes, although quality characteristics of a cord can be altered in a longitudinal direction thereof, it is difficult to correctly locate each site of a cord at its specified position on a tire all the time. Therefore, in order to alter quality characteristics of a steel cord in accordance with the respective sites of a tire, it is necessary to form a tire in a state where a corresponding number of intermediate cord-rubber composite products have been prepared.
Moreover, in cord production using conventional twisters, it is difficult to reduce and stabilize levels of rotating property and straightness of cords, which significantly influence operational efficiency of a cutting process and a forming process at tire factories. Although efforts have been made on quality improvement and stabilization of the rotating property and the straightness of cords, these are still problems that have not yet been solved.